EDIT: Modern Canon changed to Modern Rock Canon
The easiest people to blame for the wussification of alternative rock are these guys. It's not entirely fair as prissy ballads have cropped up in every mainstream rock genre from the 60s on ("The Long And Winding Road", "Love Hurts", "Every Rose Has Its Thorn", etc). Alt rock wouldn't have been an exception, even if the Gallaghers had killed each other of during one of their drunken fist fights.
Commonly associated with the vague, ill-defined label of Britpop (which was applied just as equally to Blur, who sound nothing alike), Oasis cornered the alt-pop market with "Wonderwall" and, to a lesser extent, "Champagne Supernova" in '95, establishing themselves as the prime British import for the era. Their actual sound varies, but almost always maintains a kind of baroque pop meets Merseybeat vibe, owing much to them constantly ripping off other bands (which the Gallaghers write off as an homage). This is the sound that Britpop is generally used in reference to and can be seen as a direct predecessor to the current twee rock scene we're mired in.
There's really not much to say about Oasis. Most of their big singles (the two already mentioned, "Live Forever", "Supersonic", and "Don't Look Back In Anger") all have the same basic components that I mentioned just before. Their more rock-oriented songs ("Rock 'N' Roll Star" and... uh....) are barebones distorted riffs over barebones drumbeats, topped with Liam's bland vocals. Their music was formulaic and once everyone learned it they had no use for the originators.
I should point out that Oasis (or rather the engineers on Morning Glory) were one of the major instigators of the so-called 'Loudness War' of modern music. In brief, the Loudness War involves upping the peak volume of music tracks by compressing the dynamic range of the songs. This causes audible distortion when the music is played at high volumes, and decreases the contract between loud and soft sections of the music (which, ironically, was one of the major characteristic of alternative rock in the first place). This is all done to demand the attention of the listener; it's the same logic behind making advertisements louder than everything else on the television/radio, high volume commands attention. It works, but at the cost of listener fatigue and general low fidelity.
I'm not as crazed about all this as most people, but certain object lessons have started to bring me around. Consider the difference between the track "Californication" on album (http://www.youtube.com/watch?v=YlUKcNNmywk) and on an un-mastered bootleg (http://www.youtube.com/watch?v=_HZxLwoiGK4). There are even more dramatic examples (Metallica's newest album, Death Magnetic being one of the most notorious) but this is the one I'm most familiar with.